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UHC and OIII filters?


a5tarman

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Hi all, I primarily use my 10 inch dob under bortle 6/7 skies. I'm trying to understand if adding a UHC or OIII filter is worthwhile in my area. I'm hoping to add more nebulae to my observation list without driving to a dark sky site. What kind of effect could I expect. Will a filter simply make a nebula kinda sorta just barely visible with averted vision if I squint in a certain way, or can I expect more?

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The key to success with visual NB filters (and without them too, actually, but less productive) in a high light pollution location is the eyes darkness adaptation. You need to learn how to gain that adaptation to the maximum and how to maintain and regain it through all phases of astronomy activities from pointing your telescope to the target - to observing it long enough to pick details (which is similar to a CCD camera, the longer you "soak in" the more that light will reveal, eyes need longer "exposures" though depending on the observer's experience distinguishing sub-peta-photon differences :D).

With a 10" aperture you might expect to see quite a lot of interesting DSO stuff already, just stick to a higher magnification to keep the background sky glow at bay. NB filters help making the latter easier at lower magnifications too, but limiting your DSOs to gas nebulae (diffuse and planetary). Though stay assured, at 10" there are plenty accessible already with experience (cumulative number of tries). Myself, I believe that the the real joy starts from 12" only and frankly from scratch ;) )

To maintain your darkness adaptation longer when observing you might want to modify your telescope in certain ways as well to eliminate as much scattered light from your surroundings as you can (e.g. flocking, dew shield, eye guards, head cover with air venting, etc).

Edited by AlexK
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I observe under similar skies with an 8” reflector and can see plenty of planetary nebulae and emission nebulae, reflection nebulae can be more challenging. Filters can assist and I wouldn’t be without them but, even under these skies, some objects are better without, it really depends on the object, it’s magnitude and sky conditions at the time. Be assured, your scope will enable you to view thousands of objects under your skies and I would definitely invest in filters, I have Astronomik UHC, O111, Baader Moon and Skyglow and contrast booster. With or without filters a lot of objects take patience, good dark adaptation and trial and error 

Edited by Jiggy 67
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5 hours ago, a5tarman said:

Hi all, I primarily use my 10 inch dob under bortle 6/7 skies. I'm trying to understand if adding a UHC or OIII filter is worthwhile in my area. I'm hoping to add more nebulae to my observation list without driving to a dark sky site. What kind of effect could I expect. Will a filter simply make a nebula kinda sorta just barely visible with averted vision if I squint in a certain way, or can I expect more?


I also use a 10” Dob from a light polluted town. Under such conditions a great deal of deep sky observing is possible and a UHC or OIII filter is a very worthwhile accessory.

However these filters are only a help with certain types of object - examples are, supernova remnants (the Veil in Cygnus a prime target) planetary nebula M27, M76, Blue Snowball etc etc.  At this time of year M42 is a must-see.  Sometimes the improvement is subtle sometimes it’s more obvious, depends on which object.

Other DSO objects are harmed by such filters - galaxies, open and globular clusters.....

As to whether a UHC or OIII is the first one to get, my own choice is a UHC that’s generally more useful, an OIII a close second but other observers find it’s the other way round.

I’ve personally found the cheapest of the UHC are effective but the more expensive filters are a step up in performance.

Ed.

 

 

 

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I use O-III, UHC and occasionallly H-b filters from my bortle 5 garden with my 12 inch dob. The H-b is of limited use but the UHC and especially the O-III's have quite an effect on the supernova remnants, planetary nebulae and many emission nebulae.

It's worth reading up on effective exit pupils to get the best from these filters because this can make quite a difference to the overall impact.

Get it right and the illustration below is not an exaggeration:

Simulated VEIL NEBULA with and without filters. - Deep Sky Observing -  Cloudy Nights

It is worth getting good quality filters though. The cheaper ones are noticeably less effective I've found to my cost in the past :rolleyes2:

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Nebulae emit light in discrete wavelengths and any other light passing the filter is extraneous to seeing the nebula.

The more of the extraneous light suppressed, the darker the background behind the nebula and the more contrast there is between nebula and sky.

To achieve that, a bandwidth in the filter that just passes the light of the nebula is ideal.

 

But there are always slight irregularities in the production of filters, so the bandwidth needs to be about 8nm wider than the minimum possible bandwidth of 14.6nm.

So a 23nm bandwidth UHC filter would be the ideal.  It would be a universal nebula filter and work on all the emission nebulae.

Most of today's higher-quality UHC type narrowband filters are in the 22-27nm bandwidth range, like Astronomik, TeleVue, DGM, Lumicon, ICS.

A lot of the inexpensive Chinese-made nebula filters are in the 45-50nm bandwidth range, and are not effective at increasing contrast in light-polluted skies.

So since only 22-23nm of bandwidth is necessary for maximum contrast, any extra bandwidth just lets a bit too much light pollution through.

There is a little contrast enhancement, just not enough to make a large difference.

 

So, what can you do to maximize the experience?

1. Use only low powers with the filter.  On an 10" scope, a maximum of 100x.

2. Make sure you are as dark-adapted as possible.  That means 30-45 minutes outside, away from all lights, i.e. at least 30 minutes after turning off all lights.

3. Make sure the object you're looking at is at least 30° above the horizon.  Your nebula target would lose ~0.2-0.3 magnitudes of brightness at the zenith, and double that at 30°.

It could lose a whopping 2.0-3.0 magnitudes at the horizon!!  So try to view the nebula when it is near or crossing the imaginary N-S meridian.

4. Make sure the nebula is an emission-type nebula.  The nebula filters won't help reflection nebulae like those in the Pleaides or M78 in Orion.

5. Buy the size that fits your lowest power eyepieces, star diagonal, or bottom of the 2" to 1.25" adapter.

Edited by Don Pensack
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11 hours ago, a5tarman said:

Hi all, I primarily use my 10 inch dob under bortle 6/7 skies. I'm trying to understand if adding a UHC or OIII filter is worthwhile in my area. I'm hoping to add more nebulae to my observation list without driving to a dark sky site. What kind of effect could I expect. Will a filter simply make a nebula kinda sorta just barely visible with averted vision if I squint in a certain way, or can I expect more?

can i ask why you dont want to drive some were dark ?

makes a lot of difference

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17 hours ago, a5tarman said:

Hi all, I primarily use my 10 inch dob under bortle 6/7 skies. I'm trying to understand if adding a UHC or OIII filter is worthwhile in my area. I'm hoping to add more nebulae to my observation list without driving to a dark sky site. What kind of effect could I expect. Will a filter simply make a nebula kinda sorta just barely visible with averted vision if I squint in a certain way, or can I expect more?

As Faulksy says, no substitute for dark transparant skies!

I had a 10", and took a punt on an Astronomik O-III filter for nebulae, after reading Dave Knisely's survey.  I was bowled over on the Veil, and many other planetary nebs.  It will make nebs pop, increasing the contrast by selecting the narrow emission band, and attenuating most other wavelengths.  I later purchased a UHC, but I'd recommend the O-III if you were to pick one/ or a first choice to purchase.

I observe from Bortle ~4 skies, and if you have sodium street lamps, then yes filters should help from your skies.  Honestly, I'd recommend trying the O-III: it will prove a useful tool in the long run.

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6 hours ago, faulksy said:

can i ask why you dont want to drive some were dark ?

makes a lot of difference

I do want to drive to a dark sky site, but 90% of my observing will be from my backyard so I'm trying to determine what will be effective in improving my observations under light pollution. Considering the cost of filters I don't want to buy one to find out it is only useful under dark skies anyway.

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I have a late 90s Lumicon OIII that made the Veil nebula visible in exquisite detail under my Bortle 5/6 skies in my 15" Dob.  Without it, I couldn't make out any of it.  It also works well with my 8", but the bigger the better with OIII filters.

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I think the UHC and O-III filters will have a positive effect on nebulae under bortle 6/7 skies but the difference will be seeing some hints of the target as opposed to perhaps nothing at all, rather than seeing a target at it's best.

I guess the question is, what is that worth to you ?

 

 

 

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I have both a UHC and Olll filters.

The filter I use the most often is the Baader Neodymium. It is my ‘Swiss-army knife’ filter.

BTW - all my filters are 1.25” and I use a 2”-1.25” adapter ring for my one and only 2” e/p.

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I got an Astronomik UHC first, and it certainly made a difference from my Bortle 4 garden. As others have already said, with some objects like the veil, it can make the difference between something or nothing. On other objects, I found that it could add some detail but remove other parts, so changing the shape slightly. I've only recently got an O-III, and with the recent poor weather I haven't used it much. I did a comparison on M42 and both improved the view, but I couldn't see any difference between the two.

This resource is an attempt to compare the effect of different filters for specific objects (though obviously that may be affected by equipment, LP, observer's eyes).

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41 minutes ago, Zermelo said:

I did a comparison on M42 and both improved the view, but I couldn't see any difference between the two.

The effect becomes more pronounced with more light pollution due to the narrower passband of the OIII rejecting more spurious light, increasing contrast.  Losing the H-beta line doesn't seem to affect the image as much as rejecting more light pollution.

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34 minutes ago, Louis D said:

The effect becomes more pronounced with more light pollution due to the narrower passband of the OIII rejecting more spurious light, increasing contrast.  Losing the H-beta line doesn't seem to affect the image as much as rejecting more light pollution.

That makes sense.

I also rediscovered this thread that might interest OP:

 

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17 hours ago, a5tarman said:

I do want to drive to a dark sky site, but 90% of my observing will be from my backyard so I'm trying to determine what will be effective in improving my observations under light pollution. Considering the cost of filters I don't want to buy one to find out it is only useful under dark skies anyway.

If a backyard observer, with some local light intrusion and general urban light pollution, there could be varied approaches. 

Some on here like to wear a hood to block as much light as possible from their line of vision and perhaps enable some potential dark adaption to develop. If this appeals, there are plenty of threads or just start a new one, the hooded brigade will soon chip in.

A few with deep pockets have invested in Night Vision technology, visit the EEVA section. This may transform your observing sessions from a light polluted sky, if you are not a purist 'photon gatherer'. However very expensive and will now be issues importing and customs charges added from EU.

Observe deep sky objects when the sky is transparent, perhaps after a period of rain, objects to observe are high and as late as possible for optimum darkness and perhaps less local light intrusion.

On the subject of filters, perhaps a good quality 2" UHC and enjoy the Orion Nebula.

If though 10% of your time can be made at dark sky locations, this will become both complementary and rewarding. 

 

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Just be aware, an O-III filter does not work well on the large H-II region gas clouds.

A UHC works better on M42/43, M8, M20, M17, M16, hydrogen clouds in Cygnus.

An O-III filter will work better on planetary nebulae, WolfRayet excitation nebulae, supernova remnants (except M1, where a UHC works better)

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10 hours ago, Don Pensack said:

...A UHC works better on M42/43, M8, M20, M17, M16, hydrogen clouds in Cygnus...

Agreed! :icon_salut:

I can even split the ‘Trapezium’ in M42 with a UHC.

Edited by Philip R
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It's nice to be able to experiment with different filters on different targets to see the effects. Alternating between no filter, a UHC, an O-III and an H-B on Messier 42 for example shows some really interesting differences in extent, the enhancement and sometimes the diminishment of the various parts of this complex target :smiley:

 

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I've never used a UHC but I've found that my Lumicon OIII is an occasionally useful tool to have in the box. I don't use it very often, as almost everything I look at with my 10" can be seen well enough without it, and very small planetary nebulae which look like a fuzzy blob aren't dramatically altered in appearance by bothering to put a high contrast filter on. But things like the Veil Nebula and Owl Nebula are just simply invisible to me without one, and they're well worth seeing. Also the other night I was looking at M42 at just 38x, so the background sky was quite bright, but there's no question that putting the OIII dimmed it very significantly and brought out so much more detail.

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11 hours ago, John said:

It's nice to be able to experiment with different filters on different targets to see the effects. Alternating between no filter, a UHC, an O-III and an H-B on Messier 42 for example shows some really interesting differences in extent, the enhancement and sometimes the diminishment of the various parts of this complex target :smiley:

 

A great comment!  It points to the enhancement of different details with different filters.  That's especially true of M42/43.  Even a broadband filter like the UHC-S is wonderful on that object.

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